In a small bowl mix 1/2 cup chicken broth, beef, lamb, rice, onion, pine nuts, parsley, garlic and mint. Set aside.
Gently rinse grape leaves and pat dry with paper towels. Arrange on a work surface with the shiny side down. Place 1 tablespoon meat mixture in the center of a grape leaf. Tuck in ends and roll tightly toward the leaf point. Repeat until meat mixture and leaves are used up.
Arrange wrapped leaves, in layers, in a large sauce pan with seam side down. Cover leaves with remaining broth and the lemon juice. Weight leaves with a heavy plate to prevent opening.
Cook over low heat 45 minutes.
Serve with plain yogurt; garnish with lemon slices.